The amount "free sugar" contributes to dietary calories should be slashed by half, according to scientists advising the British Government.

A draft report from the Scientific Advisory Committee on Nutrition (SACN) urges a reduction from the current recommendation of 10% of dietary energy intake to 5%.

This is the equivalent of 25 grams of sugar for women and 35 grams, or seven to eight teaspoonfuls, for men.

A single 330ml can of fizzy drink contains around seven teaspoonfuls, and so would meet this limit on its own.

Professor Ian Macdonald, who chaired the SACN carbohydrates working group, said: "One can will use up your 5%, but the message has got to be to reduce consumption of all sorts of foods and drinks.

"It is possible without it being a boring Second World War type of diet."

The guidance will go out for public consultation before a final report due to be published in March 2015.

Free sugars are those added to food or contained in fruit juices, honey, syrups and sweetened drinks. They do not include the sugars locked inside fresh fruits and vegetables, and milk.

The scientists also recommended an increase in daily total fibre intake from around 18 grams to 30 grams for adults.

Evidence shows that fibre improves bowel health and can help prevent heart disease.

Carbohydrates should make up half the population's daily calorie intake, as recommended at present, said the committee.

The nutrition experts came to their conclusions after reviewing thousands of research papers from scientists investigating the health effects of sugar.

Too much sugar in the diet is known to be one of the primary reasons for the obesity epidemic and rising rates of Type 2 diabetes. Free sugar also poses a serious threat to dental health.

Dr Ann Prentice, chair of the SACN and director of human nutrition research at the Medical Research Council, said: "SACN has carried out a rigorous assessment of an extensive evidence base on the role of carbohydrates in the diet and their relationship to health.

"There is strong evidence in the report to show that if people were to have less free sugars and more fibre in their diet they would lower their risk of cardiovascular disease, Type 2 diabetes and bowel cancer.

"For their health, people need to consume a balanced diet which includes carbohydrate-rich foods that are low in free sugars and high in fibre."

Public Health England (PHE) said it was rising to the sugar challenge by ramping up its health campaigns and calling on consumers, food companies, retailers and other stakeholders to collaborate in tackling the issue.

Dr Alison Tedstone, chief nutritionist at Public Health England, stressed that for the time being, the current recommendation that free sugar should make up 10% of daily calories would remain in force.

She said: "We are very concerned about sugar intakes.. If you look at the dietary data you see that every single population group in England is exceeding the recommended intake for sugar.

"One of the reasons we're worried is because of obesity. We tend to think of obesity as being a cosmetic problem, only to do with those images we see of grossly obese people. It's not. Being a little bit overweight or podgy increases your risk of metabolic diseases.

"The situation in the UK is very difficult. We have over two thirds of the adult population being overweight or obese. One in five four to five-year-olds are obese or overweight. By the time they leave primary school, one in three are obese or overweight."

She added that 28% of five-year-olds suffered from tooth decay in England.

Soft drinks account for about 30% of children's sugar intake, said Dr Tedstone.

Manufacturers' claims that sweetened drinks only make up some 3% of sugar consumption were based on the whole population, not just children.

Speaking at a news briefing in London, Dr Tedstone said cutting sugar to 5% of calories simply meant making a lifestyle choice to limit consumption of food items such as fizzy drinks, chocolate, sweets, and cakes.

She insisted: "It doesn't mean having a very different diet. It means thinking about swapping these high-sugar foods for low-sugar alternatives; for example, drinking water.

"Instead of having a chocolate bar you could have a piece of fruit."

Professor Graham MacGregor, chairman of the group Action on Sugar, said: "We are currently consuming far above the current recommendations of 10%, so it is fantastic that this strong recommendation has been put forward and shows the urgency of acting now.

"He must start by setting targets for reducing sugar in soft drinks this summer and move responsibility for nutrition to an independent body such as the Food Standards Agency so that the soft drinks and food industry are given a level-playing field, with the threat of regulation to ensure the whole of the food industry comply."

Barbara Young, chief executive of Diabetes UK, said: "This report highlights the worrying role sugar is playing in fuelling the high obesity rate that has, in turn, led to the sharp rise we are now seeing in Type 2 diabetes.

"We welcome the fact that this report highlights how having too many high-sugar foods and drinks is clearly bad for you. But, while we need to raise awareness about the negative effect a high-sugar diet can have on your health, we must not lose sight of the damage that high-fat foods can cause."

Sue Davies, food expert at the consumer watchdog Which?, said: "With rising obesity rates, more action needs to be taken by the food industry to lower sugar levels and there should also be greater responsibility when promoting products that are high in sugar.

"More manufacturers should follow the lead of retailers by committing to traffic- light labelling, so that it's clear how much sugar products contain, allowing consumers to make an informed choice."

Changing the sugar guidance to 5% of daily energy intake would bring the Department of Health in line with the World Health Organisation, which already makes this recommendation.

Leading nutrition expert Professor Susan Jebb, from Oxford University, said: "Dietary recommendations are set on a nutrient basis, but consumers need to put this information in the context of the foods they eat and their overall eating pattern.

"There is no suggestion that sugary foods should be replaced in the diet by fatty foods. Water should be the drink of choice and we need to eat less confectionery and fewer biscuits and cakes. This will cut calories, fat and sugar and bring holistic health benefits."

Dietician Catherine Collins, from St George's Hospital NHS Trust in London, said: "For many the calorie content of sugar-sweetened drinks as a contributor to their struggle with weight is never considered, yet these incidental calories consumed for thirst contribute significantly to total daily sugar and calorie intake. Sugary drinks alone provide a third of the total sugar intake in the average teenager's diet.

"I'd like to see a complete ban on broadcast advertising of high sugar drinks. With all the major companies producing calorie (and therefore sugar) free options, these should become the only version to be the 'advertised' norm."

"Studies show that the first bite of chocolate is the one that gives you greatest satisfaction, so no need to run to extra expense and calorie load with that 100 gram bar," said Ms Collins. "Small is beautiful when it comes to sweet treats."

Simon Gillespie, chief executive of the British Heart Foundation, said: "Given the links between a high sugar diet and obesity it's vital we take action to address this ever-pressing issue.

"For too long people have been eating too much sugar. Getting people to lose their sweet tooth won't be easy and education alone won't be enough to get us to kick this habit.

"We need to stop food manufacturers bombarding families with advertising promoting products high in sugar, fats, and salt. The Government should introduce a 9pm watershed ban and consistent regulation across all media to protect children from being tempted into unhealthy choices.

"A sugar tax on sweetened drinks could also help us make major inroads into tackling this serious public health concern."

British Sugar, which produces more than one million tonnes of sugar in the UK each year, said it was "surprised" by the draft recommendation to halve free sugar consumption at a population level.

Managing director Richard Pike said: "In reality this sets a level that will be very difficult for most people to meet. For example, this would be the equivalent of consuming the sugars found in a small glass of orange juice and a 125 gram yoghurt."

He pointed out that the report drew a distinction between sugar consumption per head of population and individual sugar intake. For individuals, it recommended that free sugars should contribute no more than 10% of total calories.

"This runs the risk of confusing consumers even further," said Mr Pike. "Tackling the obesity epidemic requires collaborative action to help consumers better manage and balance their overall calorie intake, diet and lifestyle. We are committed to playing our part."

Sian O'Shea, from the British Dietetic Association, said: "As a population, we are consuming too much sugar, with the average intake for children being around 15% of their overall total energy intake per day and around 12% for adults. The current recommendation is for this figure to be no more than 11% (10% if alcohol is excluded).

"Concern over sugar in the diet, and particularly in sweetened soft drinks, has been growing as well as the rising rates in obesity and obesity-related diseases such as Type 2 Diabetes and cardiovascular disease.

"We strongly urge the food industry to support the promotion of healthier lifestyles and to take action by reducing sugar content in their products."